Walnut Blight: Genetic Variation in English Walnut in Response to Inoculation with Xanthomonas Campestris Pv. Juglandis

This was the second year of a study which evaluated the genetic variation among English walnut germplasm in response to infection by Xanthomonas campestris pv. juqlandis (Xcj), the bacterium which causes the disease walnut blight. Immature leaves and nuts of 10 walnut accessions representing the breadth of currently available germplasm were inoculated. Leaves were scored for disease severity at 21 and 35 days post inoculation (d.p.i.). Nuts were inoculated at about two and six weeks past full bloom, and evaluated for rate of nut abscission and the condition at harvest of the shell and kernel. The results of the 1989 leaf inoculations confirm and amend data obtained in 1988. Significant inter-cultivar differences in lesion size, number and leaf abscission were detected. Nut inoculations in 1989 revealed significant among-cultivar differences in the rate of abscission of infected nuts. These differences were consistent across a range of nut sizes. In general, two types of responses to nut infection were observed. Most tested cultivars (including 'Hartley', 'Sunland', 'Serre and 'Franquette', abscise small (0.5-2.0 em) infected nuts within 30 days of infection. Other accessions (such a~ 'Sinensis #5' and 'Adams') retain infected nuts. Most of these retained, infected nuts matured and contained light colored kernels at harvest, depending on their size at infection. Results suggests a correlation between the retention of infected nuts and the formationof a wound peridermaroundblight lesions. This periderm may playa role in restrictingthe spread of blight through nuts.